Glaucoma is a disease where the intraocular pressure is elevated. It affects significant numbers of our population. The treatment of glaucoma is usually medical, however, medications often fail to control some forms of glaucoma. When further treatment is required a microsurgical operative procedure is performed. This procedure involves constructing a fistula or opening in the tissue wall of the sclera to enhance fluid flow from the internal portion of the eye (ciliary body) which secretes the fluid (aqueous humor) through the newly formed opening. This opening is typically made in a cutting type of procedure. This involves incising the external ocular tissues (conjunctiva) and dissecting the scleral tissues. This dissection results in attendant risks including bleeding, and development of extremely low intraocular pressure or hypotony. Post operative care is prolonged due to the large size of the scleral incisions, the possibility of complications and variability in wound healing. The procedure is generally referred to as a filtering operation, as a trabeculectomy, sclerectomy or lamellar scleral flap procedure. In a so-called full thickness fistulization procedure, a hole of a diameter of 2-4 mm is formed through the sclera. In a so-called partial thickness fistulization, an opening in the form of a slit of 100-300 microns extends through the tissue wall into the anterior chamber and an ostium or aperture of 2-4 mm extends from the slit at the posterior surface of the tissue wall. The trabeculectomy, partial thickness sclerectomy or fistulization, has become the prevalent procedure since a valve effect occurs when a partial thickness aperture is present. Many of the immediate post operative complications of surgery are reduced by the more controlled outflow achieved by this type of surgery.
Recently, lasers have been used to perform fistulizing procedures. These laser procedures are currently used to produce a full thickness fistula by a procedure referred to as laser sclerostomy. This has been performed with holmium, YAG, erbium and other laser penetrating means. In the current mode, (ab externo), a small incision is made after a subconjunctival injection of air or other fluid has been introduced into the subconjunctival space. This fluid allows the laser probe to pass beneath the conjunctival tissue without "button-holing" the tissue. The laser probe is then introduced into the subconjunctival space. The probe is advanced to the sclera proximal to the limbal area of the eye. The laser energy is directed from the laser probe toward the sclera until the energy produces a fistula through the full thickness of the sclera. Once this occurs the laser probe is removed and the initial conjunctival incision is sutured and the procedure is complete.
In the ab interno version of filtering surgery, laser energy is aimed using a contact lens (goniolens) to produce a full thickness opening in the sclera. A mechanical method of producing a fistula using a rotating, cutting blade (trabecuphine) also results in a full thickness opening.
The ab interno fistulizing surgery, by lasers or cutting, suffers, in its present state, from several disadvantages. Ab externo laser surgery has produced only full thickness fistulas. This results in attendant problems with hypotony, choroidal effusion, choroidal hemorrhage and shallowing of the anterior chamber of the eye. The fistula also frequently closes. Modifications of the procedure employ either intra-operative or post-operative injections of anti-scarring agents (antimetabolites) to improve the results. The small size of the fistula (100-300 microns) may be an advantage for controlling fluid flow, but the long term success of such small fistulas may be temporary since they scar down more easily than the larger (2-4 mm) fistulas in guarded filtering surgery (trabeculectomy) by cutting. Larger fistulas in a full thickness procedure (sclerectomy, thermal sclerostomy) produce a greater frequency of complications than guarded procedures.